Francisella tularensis is a highly infectious gram-negative coccobacillus that causes the zoonosis tularemia and is a Category A agent. The need for understanding the molecular basis for F. tularensis disease in order to combat possible threats is evident. A hallmark of tularemia is the ability of the bacterium to grow in mammalian hosts before the onset of a protective cell-mediated immune response. Mammalian hosts are endowed with numerous antimicrobial effector functions. Accordingly, F. tularensis has evolved mechanisms to subvert host defenses. It is very striking that this small bacterium can infect its host via a variety of different infection routes, each of which involves a different host tissue site with a vastly different microenvironment. Given that F. tularensis is so successful at infecting its host via multiple tissue sites, our hypothesis is that in addition to a core set of genes that are needed for general survival and growth in vivo, F. tularensis possess additional genes that are required in specific tissues or microniches. Thus, our overarching goal is to identify novel core and tissue-specific virulence factors in F. tularensis. In the first aim, we will identify tissue-specific (e.g. lung-, spleen, and skin-specific) F. tularensis virulence factors using our well-established microarray-based negative selection methodology following intranasal, intraperitoneal and intradermal routes of inoculation. In the second and third aims, we will validate the tissue-specificity of novel virulence factors and characterize the molecular mechanisms in our mouse models of infection and in vitro in tissue culture assays. This project is synergistic with the other Francisella project in the Program in that it will allow us to directly compare the results of genetic and proteomic analyses obtained by Dr. Marcus Horwitz's laboratory utilizing F. tularensis subsp. tularensis, the LVS and F. novicida with our in vivo negative selection results. Since we will be using the same transposon mutant library for our in vivo and in vitro assays, followed by our very rapid microarray-based detection method, we will identify novel Francisella factors that interact with host proteins in an extremely efficient manner.